1. Field of the Invention
The present invention relates to a high frequency oscillator circuit, a phase-locked loop circuit (referred to as a PLL circuit hereinafter) using the high frequency oscillator circuit, a semiconductor device using the PLL circuit, and a communication apparatus including the semiconductor device. In particular, the present invention relates to a high frequency oscillator circuit for generating a high frequency signal such as a microwave signal, a quasi-millimeter wave signal or a millimeter wave signal, utilizing two field effect transistors with a feedback circuit of a short-stub transmission line and one field effect transistor, and further relates to a PLL circuit using the high frequency oscillator circuit, a semiconductor device using the PLL circuit, and a communication apparatus using the semiconductor device.
2. Description of the Related Art
FIG. 12 is a circuit diagram of a high frequency oscillator circuit 201 according to a first prior art. The high frequency oscillator circuit 201 is an oscillator circuit using a cross-coupled type multivibrator and, as shown in FIG. 12, is configured to include two field effect transistors 51 and 52, a capacitor C1, inductors L1 and L2, and a direct-current voltage source 22, and then, achieves an oscillating operation by developing a negative resistance.
FIG. 13 is a circuit diagram of a high frequency oscillator circuit 202 according to a second prior art. The high frequency oscillator circuit 202 is a gate and source feedback type oscillator circuit, and as shown in FIG. 13, it is configured to include a field effect transistor 1, two transmission lines 11 and 12 (which are generally microstrip lines or coplanar lines on a semiconductor substrate or a dielectric substrate) constituting short-stub feedback circuits, respectively, a high frequency blocking inductor Lc1, and two direct-current voltage sources 21 and 22, and then, achieves an oscillating operation by developing a negative resistance. The high frequency oscillator circuit 202 performs the oscillating operation in the case where an input impedance Zin of the field effect transistor 1 when seen from an output terminal To connected to a drain of the field effect transistor 1 satisfies the following expressions (1) and (2):Re(Zin)+RL<0  (1), andIm(Zin)=0  (2),
where Re(•) denotes a real number part of an argument and Im(•) denotes an imaginary part of the argument, and these will apply hereinafter. In addition, RL denotes a load resistance value.
FIG. 14 is a graph showing simulation results of the high frequency oscillator circuit 201 shown in FIG. 12 and showing a normalized oscillation frequency relative to a Q-value of the inductor. A reference oscillation frequency for normalization is 50 GHz. As apparent from FIG. 14, the high frequency oscillator circuit 201 is disadvantageously required to include the inductor having a relatively high Q-value.
Furthermore, the high frequency oscillator circuit 202 shown in FIG. 13 has the following problems. A line length of each of the transmission lines 11 and 12 connected to the gate and source of the field effect transistor 1, respectively, is relatively large (e.g., about 1.5 millimeters (mm) in a 30-GHz oscillator circuit using microstrip lines having a width of 14 micrometers (μm)). An area of the high frequency oscillator circuit 202 is made large, a loss of each of the short-stub feedback circuits increases, and a Q-value of the circuit decreases.
Moreover, the Japanese patent laid-open publication No. JP-7-221545-A discloses a cascade-coupled type voltage-controlled oscillator capable of pulling out sufficiently high negative resistance characteristic from the circuit and having strong oscillation power. The voltage-controlled oscillator is a cascade-connected voltage-controlled oscillator in which a direct-current flowing from a power source is applied first to a transistor of an oscillation amplifier unit and then to a transistor of a buffer amplifier. Furthermore, a resistance that decides the direct-current of the voltage-controlled oscillator is connected to the transistor of the buffer amplifier outside of a feedback loop of the oscillation amplifier unit. Due to this, the resistance that decides the direct-current of the voltage-controlled oscillator does not act as a loss resistance of the feedback loop of the oscillation amplifier unit. Therefore, sufficiently high negative resistance characteristic can be pulled out from the voltage-controlled oscillator, and this leads to that the cascade-connected voltage-controlled oscillator can output strong oscillation power. The voltage-controlled oscillator is configured so that not the feedback circuit but the transistor of the oscillation amplifier unit is connected to a source of the transistor of the oscillation amplifier and in which an oscillation signal is outputted from a drain of the transistor of the oscillation amplifier unit.
The Japanese patent laid-open publication No. JP-8-107309-A discloses a voltage-controlled oscillator circuit capable of dealing with low frequency noise output from a power source circuit of the voltage-controlled oscillator circuit without any connection of a ripple filter or the like to an outside circuit of the voltage-controlled oscillator circuit. The voltage-controlled oscillator circuit is characterized in that a capacitor that grounds a base of an amplification transistor of a buffer amplifier circuit unit in a low frequency alternating current manner, the buffer amplifier circuit is allowed to be regarded as a ripple filter for the low frequency noise added to the buffer amplifier circuit unit from the power source circuit and is allowed to perform ordinary amplification operation for a signal applied from an oscillator circuit unit. The voltage-controlled oscillator circuit is configured so that not a feedback circuit but a transistor of the oscillator circuit unit is connected to a source of the amplification transistor of the buffer amplifier unit.
The Japanese patent laid-open publication No. JP-10-209752-A discloses an oscillator circuit constituted as a microwave integrated circuit so as to be able to obtain optimum oscillation states with selected frequencies even if a resonance frequency of a resonance circuit is selected over a wide frequency range. In the oscillator circuit constituted as a microwave integrated circuit, an external terminal is arranged at a gate of an oscillation field effect transistor, and a resonance circuit is connected to the external terminal. Further, a series-connected circuit in which a first feedback capacitance and a first switch field effect transistor are connected in series and a series-connected circuit in which a second feedback capacitance and a second switch field effect transistor are connected in series are arranged in parallel between a source of the oscillation field effect transistor and a ground. By turning on or off the two switch field effect transistors at a gate-controlled voltage, it is possible to change a feedback capacitance value to, for example, three types of values and to obtain oscillation states optimum for three types of resonance frequencies. The oscillator circuit constituted as a microwave integrated circuit is configured so that a frequency selection switch field effect transistor is connected to the source of the oscillation field effect transistor.
Each of the oscillator circuits disclosed in the Japanese patent laid-open publications Nos. JP-7-221545-A, JP-8-107309-A, and JP-10-209752-A has the following problems. It is difficult to make an area of the oscillator circuit smaller, and it is difficult for the oscillator circuit to oscillate with a higher oscillation frequency.